The long-term goals of this laboratory are to elucidate mechanisms of intestinal disease in infants and children and to use basic science to identify rational treatments. Relevant to the mission of NIH, this proposal is designed as a pilot study that will generate and expand the base of in vivo data and direct future research on probiotic therapeutic interventions. Additionally, the investigators aim to utilize state-of-the science technologies to determine the constituents of complex gut microbiotia that are associated with and may contribute to colic, and to examine the impact of probiotics on the composition of the bowel flora. Colic is defined as crying and fussiness for >3 hr daily for >3 d weekly in a child <3 months old. Currently, there is insufficient evidence supporting any specific treatment to relieve symptoms of infants with colic, with the possible exception of hypoallergenic formula. However, there is evidence that infants with colic produce too much hydrogen gas in the intestine and that their fussiness may be improved by daily administration of certain probiotic (health-promoting) bacteria. Recently, studies in adults with irritable bowel syndrome showed significant improvement with probiotic therapy. Our hypothesis is that infants produce more hydrogen gas because of a different colonic bacterial flora and that modification of this flora with Lactobacillus rhamnosus GG will alter gas production, thereby reducing the time these infants spend crying. We choose to study Lactobacillus rhamnosus GG, one of the two probiotic strains which has received the most investigation in babies with diarrhea. L. rhamnosus GG is FDA-approved and available over-the-counter in the U.S. Relevance to public health: Colic affects at least 8% of all infants. Our studies would not only provide evidence for whether oral L. rhamnosus GG helps reduce the number of minutes each day with colic;they would also provide evidence of a possible mechanism. Specifically, we will demonstrate if L. rhamnosus GG treatment of infants with colic who have excessive breath hydrogen levels will change the bacterial community in the colon and reduce fecal markers of inflammation. In the process, we will also demonstrate if cultivation-independent analyses of stool will reveal bacterial species associated with colic that were not previously recognized.